James Webb captured a near-perfect Einstein ring

An observation that is very rare today, but could become much more common thanks to the exceptional performance of the telescope.

The James Webb Space Telescope has recently emerged with a remarkable new image; he impressively surprised with a galaxy whose glow took 12 billion years to reach us. What makes it interesting is that it appears to us in a rather special form, namely an almost perfect Einstein ring.

This is not the first time this galaxy, called SPT-S J041839-4751.8, has been photographed. The venerable Hubble and even Webb have already drawn his portrait several times. But recently JWST looked back at it to examine it with another instrument, its Mid-Infrared Instrument (MIRI).

According to ScienceAlert, the raw data has since been published on the MAST portal. They were then recovered by Spaceguy44, a resident of the Reddit platform who also happens to be a PhD student in astronomy. He compiled these elements to produce a nice composite image, seen by ScienceAlert; it highlights the most striking feature of this little corner of the sky, Einstein’s famous ring.

©NASA/ESA/Spaceguy44 via Reddit

What is an Einstein Ring?

In the center of the image we can see a light source which appears to be enclosed in the center of a strange circular halo. They are actually two very separate galaxies and very far apart.

The blue dot in the center of this structure corresponds to the galaxy closest to us. The other is located far behind, but exactly on the same axis – a bit like the Moon and the Sun during a total eclipse, but at a much greater distance.

One would therefore expect the second, namely SPT-S J041839-4751.8, to be completely obscured by the first. However, it is clearly visible; it even appears as an almost perfect circle. And to understand the origin of this phenomenon, we must quickly look at the general theory of relativity formalized by Einstein – hence the name.

This theory states that objects generate a gravitational pull proportional to their mass. In the case of extremely massive objects such as galaxies, this generates a significant deformation of space-time in the vicinity of the celestial body in question.

And when light passes through this circumference, its path deviates to follow the curvature of spacetime. A finding that gave Einstein food for thought; indeed, in his works that paved the way for general relativity, we find a mention of a “gravitational lens”.

A “lens” on a galactic scale

In optics, a lens is a transparent device whose surfaces have a carefully calibrated curvature. They make it possible to manipulate the path of the light rays with great precision thanks to the phenomenon of refraction.

Functionally, gravitational lenses are quite similar to physical lenses; although they involve different physical principles, astronomers can also use them to observe otherwise invisible objects with a good level of precision. It is because of this phenomenon that SPT-S J041839-4751.8 appears to us as a very visible and well-defined circle.

The phenomenon of gravitational lensing schematically by ESA. The grid represents the curvature of spacetime. © NASA, ESA & L. Calçada

Note that this term should be taken with a grain of salt because of a very important difference; unlike a standard lens, a gravitational lens does not have a focal point, but a focal axis. This means that the distance between the telescope, the “lens” and the object to be observed does not matter in this case.

But this is still a very rare sighting, and for good reason; yet the three objects must be perfectly matchedwhich is quite unlikely given the distances involved.Einstein himself thought that it would be impossible to get close enough to the center line and that we would probably never have an instrument with sufficient resolution for the observation… but JWST decided otherwise!

And without this gravitational lensing effect, SPT-S J041839-4751.8. would probably separable ; at best it would take the form of a small point of light from which it would be difficult to extract any meaningful data.

In any case, it is a new image that will decorate the already well-stocked hunting table in JWST. And given the outstanding contributions this engineering marvel has already given us in just a few short months, it’s entirely possible that observations of Einstein’s rings will become more frequent in the future.

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